I picked up a Carver M400 cube the other day. it was blowing fuses. Found a service manual online that is very detailed! following the steps in the manual i found out it had a blown Bridge rectifier for the 25V supply, got that repaired and now i can get it to the point where i raise the voltage on a variac and the triac starts to fire but it is running very rough. looks like the triac is not firing correctly. from what i gather in the SM that at idle I should only see half of each sine wave. that the triac should be cutting off at the peak of each. and what im seeing looks more like a full sine with a glitch around crossover every other sine and the transformer ticks.
changing the triac made no difference so i am assuming that the diac is bad. I cant get it past 60% of line voltage without it drawing lots of current. and adjusting the firing control pot on the PSU board doesn't seem to do anything which i figure is probably normal at this point.
I'm curious why i couldn't use a standard light dimmer adjusted to half sine to fire the transformer just for testing? there wouldn't be any regulation but I could at least test to see if the rest of the amp comes up and runs long enough to verify the problem is in the triac control section. Looking at the schematic that's all this PSU is, is a light dimmer circuit. same exact circuit with the exception of an LDR in place of the pot.
Zc
changing the triac made no difference so i am assuming that the diac is bad. I cant get it past 60% of line voltage without it drawing lots of current. and adjusting the firing control pot on the PSU board doesn't seem to do anything which i figure is probably normal at this point.
I'm curious why i couldn't use a standard light dimmer adjusted to half sine to fire the transformer just for testing? there wouldn't be any regulation but I could at least test to see if the rest of the amp comes up and runs long enough to verify the problem is in the triac control section. Looking at the schematic that's all this PSU is, is a light dimmer circuit. same exact circuit with the exception of an LDR in place of the pot.
Zc
Before you pull out the diac , you can check voltage of Q3 collector to see it can be varied by adjusting PR1 and re check sw1 position if your unit has it. If you want to test amp circuit you need to use Vriac ( not the light bulbs) and short the triac or low voltage power supply to feed +-23v rails. Because M400 has PS caps in serial (it stacked mid rail caps on the top of low level rails) for mid rails, its PS circuit is not stable and saw more than few bridges on mid and low level rails failed. You want to seperate PS filter caps and for each rails.
well after much testing tonight i figured out that there are TWO 25V bridge rectifiers. one on the backside of the board and one on the front side. they are in parallel. and while i had replaced the rear rectifier that was shorted. I did not know there was a front rectifier and I failed to check the combination of all leads and in fact the front side rectifier was shorted too! SO....an hour to disassemble and clean the massive amounts of factory applied silicone goop off of everything. I replace the double 2 amp bridge rectifier nonsense with a nice 10 amp heavy duty rectifier. Put it back together and now i can get the amp up to full power and it passes audio.
but. I kept smelling something hot but i never could find it. I figured maybe it was just more silicone goop burning off a hot resistor someplace. and i noticed that sometimes it would not fire up. then i noticed the 50V rail caps were really hot to the touch! So i probably need to replace all the caps. I was finding that there is 40V across the 2nd tier caps (the 50v rails) but they should only have 25 across them as they are stacked on top of the 25V rails...so something is still goofy.
but. I kept smelling something hot but i never could find it. I figured maybe it was just more silicone goop burning off a hot resistor someplace. and i noticed that sometimes it would not fire up. then i noticed the 50V rail caps were really hot to the touch! So i probably need to replace all the caps. I was finding that there is 40V across the 2nd tier caps (the 50v rails) but they should only have 25 across them as they are stacked on top of the 25V rails...so something is still goofy.
I used to repair those! As I remember, the diac and opto would fail more often than the triac.
Rita (I think this is the same one) was probably the best person to troubleshoot amplifier problems. Looks like she is still in the business. You might email/contact her.
http://www.ritasvintageaudio.net/
(This site is down, right now. I found it just the other day and it looks like a nice operation!)
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It was like a 32 V diac. Nothing special. The thing that burnt up the amplifier boards is if the biasing transistor went bad. Hard on the variacs that we used to power them up for the first time or for troubleshooting.
Good luck!
Rita (I think this is the same one) was probably the best person to troubleshoot amplifier problems. Looks like she is still in the business. You might email/contact her.
http://www.ritasvintageaudio.net/
(This site is down, right now. I found it just the other day and it looks like a nice operation!)
-
It was like a 32 V diac. Nothing special. The thing that burnt up the amplifier boards is if the biasing transistor went bad. Hard on the variacs that we used to power them up for the first time or for troubleshooting.
Good luck!
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I have talked to Rita and her office manager Lynda several times. Im quite impressed with her knowledge but lack of parts seems to be the biggest issue for them/these amps.
I got the amp to a point last night where i could power it up and see that both channels would pass audio. both channels seem to clip evenly and I thought everything was ok. So i reassembled it and tried to power it up and this was the first time I had the front display board back on the amp. when i powered it up the display would dimly flicker and the amp wouldn't power up. i thought i had maybe miss aligned the front panel. its quite tricky as there are pins from the display board that need to perfectly line up with a connector on the PSU board. I thought maybe i didn't get a pin in the connector or something even thought i was very careful.
I pulled the front off and powered it up again this time on the variac and she came right up. but i noticed that the AC line current draw was higher then previously. Now about 2 amps at idle. which i thought was odd. I powered down, checked for binding wires or shorts or anything out of the ordinary and didn't find anything. so i powered up again on the variac. and it wouldn't come up. power down. wait power up, no luck, check double check repeat and it comes up.
So this time while its running I check the RP1 voltage setting and check the triac to make sure its firing correctly, check for dc on the speaker outputs, it passes signal and everything appears good but i notice its humming a bit more. look at the AC line current draw and its up around 4 amps now and something is starting to smoke. kill the amp and remove the cover again. that's when i notice that the +50V rail cap is screamin hot! hmmm
Disassemble, pull the +50V cap and measure it and yup within 10% of 2200uf, its a 25V cap so i power it with my regulated DC supply to 20V and little or no current drawn ok, let it sit for 5 minutes and its staying cool...hmm ok, grab a small transformer some alligator clips and a bridge rectifier and run it and everything is good...hmmm doesn't look bulged...dangit i need an ESR meter...but...out of curiosity i put it back in the amp and connect my meter across it. I should have 25V
and BTW...its a 25V rated cap!! zero headroom! But, bring up the amp and i have over 40V across it when i finally get the amp to refire!!!
At that point it was really late and i was tired i shut everything down and went home. Will try again today. I'm wondering if there isn't a shorted cap someplace. if the neg side cap is bad and was shorted or almost shorted to ground, that would shift the positive side up...hmmmm maybe....something is not right. I will dig back into it today and see what i can figure out. In any case the thought of having to pull all those transistors out again and deal with all that goop once again...uggh, what a pain!
Dave
I got the amp to a point last night where i could power it up and see that both channels would pass audio. both channels seem to clip evenly and I thought everything was ok. So i reassembled it and tried to power it up and this was the first time I had the front display board back on the amp. when i powered it up the display would dimly flicker and the amp wouldn't power up. i thought i had maybe miss aligned the front panel. its quite tricky as there are pins from the display board that need to perfectly line up with a connector on the PSU board. I thought maybe i didn't get a pin in the connector or something even thought i was very careful.
I pulled the front off and powered it up again this time on the variac and she came right up. but i noticed that the AC line current draw was higher then previously. Now about 2 amps at idle. which i thought was odd. I powered down, checked for binding wires or shorts or anything out of the ordinary and didn't find anything. so i powered up again on the variac. and it wouldn't come up. power down. wait power up, no luck, check double check repeat and it comes up.
So this time while its running I check the RP1 voltage setting and check the triac to make sure its firing correctly, check for dc on the speaker outputs, it passes signal and everything appears good but i notice its humming a bit more. look at the AC line current draw and its up around 4 amps now and something is starting to smoke. kill the amp and remove the cover again. that's when i notice that the +50V rail cap is screamin hot! hmmm
Disassemble, pull the +50V cap and measure it and yup within 10% of 2200uf, its a 25V cap so i power it with my regulated DC supply to 20V and little or no current drawn ok, let it sit for 5 minutes and its staying cool...hmm ok, grab a small transformer some alligator clips and a bridge rectifier and run it and everything is good...hmmm doesn't look bulged...dangit i need an ESR meter...but...out of curiosity i put it back in the amp and connect my meter across it. I should have 25V
and BTW...its a 25V rated cap!! zero headroom! But, bring up the amp and i have over 40V across it when i finally get the amp to refire!!!
At that point it was really late and i was tired i shut everything down and went home. Will try again today. I'm wondering if there isn't a shorted cap someplace. if the neg side cap is bad and was shorted or almost shorted to ground, that would shift the positive side up...hmmmm maybe....something is not right. I will dig back into it today and see what i can figure out. In any case the thought of having to pull all those transistors out again and deal with all that goop once again...uggh, what a pain!
Dave
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Have you replaced all caps yet? If it has stock caps, you want to replace with mordern caps. And you want to upgrade and un-stack 50v rail caps to 63v with same capacity and ground them. You want to check for commutators for shortage and also back flow preventing diodes ( ones connect PS to amp circuit). Your amp can still play with defective parts. You might want to check bias and local bias/protection transistors ( ones that shorts drivers and output pairs) before you fire her up. you need to short triac when you connect your amp to Variac and feed her with 1/4 of AC voltage.
Good luck to you
Good luck to you
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Caps are all stock, the whole amp is/was stock. I would bet the caps are shot and that's what took out the 2amp rectifiers.
Looks like i need to add to my parts order list!
Zc
Looks like i need to add to my parts order list!
Zc
Ok well it seems the previous tech had the wires on the transformer wrong. I jumpered the triac and brought the variac up to 30Vac and checked voltages on the transformer and they had the 50 and 80 volt lines backwards. swapped them around and now all the voltages are correct and it will sometimes run.
If i power it up cold with a flip of the switch, it runs, but if i shut down was 2-3 seconds and try and power back up it wont come on. seems i have to wait until the psu caps have drained completely before it will power up again. not sure if this is normal???
If i power it up cold with a flip of the switch, it runs, but if i shut down was 2-3 seconds and try and power back up it wont come on. seems i have to wait until the psu caps have drained completely before it will power up again. not sure if this is normal???
Is it with Variac??.
If it is with amp plugged into AC directly, this is normal. You are experiencing Caver protection circuit at work.😀
Great Work ZC
Well technically its still plugged into the variac but i am flipping the master power switch on/off and not adjusting the dial. so it is essentially the same. But..
It LIVES! working just fine now. just have to let it sit a minute or two and then it comes right back up every time. makes full power and seems to be A-OK!
other than dealing with the tons of goop that was smeared all over the output devices and inside of the case. this was fun lil amp to work on.
Zc
It LIVES! working just fine now. just have to let it sit a minute or two and then it comes right back up every time. makes full power and seems to be A-OK!
other than dealing with the tons of goop that was smeared all over the output devices and inside of the case. this was fun lil amp to work on.
Zc
Yah, this little amp is rated 250w/ch(?) with little ac adapter size transformer and mere 10lb weight. Bob Carver's amps are real fun for electronics techs. I have all kind of his amps for just fun.
Enjoy.
Enjoy.
Hi all
Resume this old thread to speak about M400 transformer; I'm wondering to replace the primary winding of a 110Vac unit to convert to 220Vac; does anyone tryed something similar?
thanks in advance
Resume this old thread to speak about M400 transformer; I'm wondering to replace the primary winding of a 110Vac unit to convert to 220Vac; does anyone tryed something similar?
thanks in advance
Hi Cal,
Ouch?
Hi NoBit,
Do you have an M-400, M-400a or M-400t? Can you give me the serial number so we can check to see what changes were factory installed? The big one is a new triac control PCB. That one does not have an impedance switch (on the rear panel).
There are two ways to convert that unit. The first is to use a step-down transformer. If you plan to run the amplifier near full power, you will need a transformer rated for 15 amperes at 120 VAC output. The second option is to get that amplifier to a shop that was factory authorized warranty for these changes. The mag coil remains the same, but much of the triac control circuits, and the triac, are changed out. Then the power supply is adjusted for the correct internal operating voltages. This is a job for factory trained people and should not be attempted by any untrained people!
Of the two options, the best for you is probably the first. The best option from a technical standpoint would be to modify the amplifier for 230 VAC operation.
This power supply generates three "tiers" of bipolar voltages. The highest tier is +/- 95 VDC, which is regulated by the triac control PCB. The main function of this circuit is to actually regulate the internal voltages against varying load conditions. If the input voltage drops, the amplifier will draw more current to maintain the same voltages inside. Keep in mind how much energy is being controlled before poking around in there.
These amplifiers are extremely compact for the amount of circuitry they contain. Take that to mean they are difficult to work on. I wouldn't open one up unless I really had to. Keep in mind that the case is also the heat sink. What that means is that the areas where it makes contact with the rear section where the transistors are mounted will have thermal compound applied. That means it is extremely messy to work on too. Just so you know, this transformer (actually a special transformer called a mag coil) does not have 120V and 220 ~ 240V taps. There is nothing like rewiring it for 230 VAC. It is not a normal transformer!
I should make one point perfectly clear about these. The triac is not used as a power on - off switch. It is used to control the amount of energy entering the mag coil. Therefore, any bright-eyed individual who decides to short the triac to force the amplifier on will cause destruction on a very wide scale. Absolutely everything will be subjected to voltages far above what is normal and probably be damaged. This damage would occur on our 120 VAC mains here. The damage from doing this in Europe would be far more complete. The amplifier would likely be too damaged to repair. Just a friendly warning in case you feel creative.
Get a giant step-down transformer. It's the most sane way to deal with this.
I have attached some pictures of an M-400 that illustrate the points I have made here. I hope they convey how tightly packed the case is, and how messy working on it will be.
Best, Chris
Ouch?
Hi NoBit,
Do you have an M-400, M-400a or M-400t? Can you give me the serial number so we can check to see what changes were factory installed? The big one is a new triac control PCB. That one does not have an impedance switch (on the rear panel).
There are two ways to convert that unit. The first is to use a step-down transformer. If you plan to run the amplifier near full power, you will need a transformer rated for 15 amperes at 120 VAC output. The second option is to get that amplifier to a shop that was factory authorized warranty for these changes. The mag coil remains the same, but much of the triac control circuits, and the triac, are changed out. Then the power supply is adjusted for the correct internal operating voltages. This is a job for factory trained people and should not be attempted by any untrained people!
Of the two options, the best for you is probably the first. The best option from a technical standpoint would be to modify the amplifier for 230 VAC operation.
This power supply generates three "tiers" of bipolar voltages. The highest tier is +/- 95 VDC, which is regulated by the triac control PCB. The main function of this circuit is to actually regulate the internal voltages against varying load conditions. If the input voltage drops, the amplifier will draw more current to maintain the same voltages inside. Keep in mind how much energy is being controlled before poking around in there.
These amplifiers are extremely compact for the amount of circuitry they contain. Take that to mean they are difficult to work on. I wouldn't open one up unless I really had to. Keep in mind that the case is also the heat sink. What that means is that the areas where it makes contact with the rear section where the transistors are mounted will have thermal compound applied. That means it is extremely messy to work on too. Just so you know, this transformer (actually a special transformer called a mag coil) does not have 120V and 220 ~ 240V taps. There is nothing like rewiring it for 230 VAC. It is not a normal transformer!
I should make one point perfectly clear about these. The triac is not used as a power on - off switch. It is used to control the amount of energy entering the mag coil. Therefore, any bright-eyed individual who decides to short the triac to force the amplifier on will cause destruction on a very wide scale. Absolutely everything will be subjected to voltages far above what is normal and probably be damaged. This damage would occur on our 120 VAC mains here. The damage from doing this in Europe would be far more complete. The amplifier would likely be too damaged to repair. Just a friendly warning in case you feel creative.
Get a giant step-down transformer. It's the most sane way to deal with this.
I have attached some pictures of an M-400 that illustrate the points I have made here. I hope they convey how tightly packed the case is, and how messy working on it will be.
Best, Chris
Attachments
Thanks Chris
I got two amps M-400 model, ser. no. 1738B and 8196B both 110Vac. One has the same transformer in your image, the other one has a black painted, but I think it is the same.
I totally agree on the triac bypassing action, what I image is that the max firing angle is near the laminated core saturation so maybe the effective "power" transmitted to the core with bypassed triac (even if the lower voltage) is too high, this is what I can image .
So if the transformer remains the same (110Vac instead of 220Vac) the only solution is to decrease by a k the firing angle, right? But the question is why Carver use a 110/220V transformer?
Last question, where I can send my amps to have the voltage/firing control work done?
Thanks again, very interesting discussion.
Eugenio
I got two amps M-400 model, ser. no. 1738B and 8196B both 110Vac. One has the same transformer in your image, the other one has a black painted, but I think it is the same.
I totally agree on the triac bypassing action, what I image is that the max firing angle is near the laminated core saturation so maybe the effective "power" transmitted to the core with bypassed triac (even if the lower voltage) is too high, this is what I can image .
So if the transformer remains the same (110Vac instead of 220Vac) the only solution is to decrease by a k the firing angle, right? But the question is why Carver use a 110/220V transformer?
Last question, where I can send my amps to have the voltage/firing control work done?
Thanks again, very interesting discussion.
Eugenio
Hi Eugenio,
I'll look them up tomorrow.
Yes, both transformers are the same. Call them "Mag coils" to differentiate those from normal transformers. Mag coils work on current. The 220 VAC option would fire much later in the cycle to limit the energy to a smaller percentage at the higher voltage. Therefore there is no need to use a different part.
I don't know who was authorized in Europe. I think they were called "Bennytone" on your side of the pond. All you can do is ask older service shops and retired technicians. There are a bunch of parts to change, the triac being one of them. You need to find a technician who has done this before. I have never had to do this modification (in reverse or otherwise).
-Chris
I'll look them up tomorrow.
Yes, both transformers are the same. Call them "Mag coils" to differentiate those from normal transformers. Mag coils work on current. The 220 VAC option would fire much later in the cycle to limit the energy to a smaller percentage at the higher voltage. Therefore there is no need to use a different part.
I don't know who was authorized in Europe. I think they were called "Bennytone" on your side of the pond. All you can do is ask older service shops and retired technicians. There are a bunch of parts to change, the triac being one of them. You need to find a technician who has done this before. I have never had to do this modification (in reverse or otherwise).
-Chris
Hi.
I've done it with my M400. It is sn1755. There are 2 types of psu. Original and new version. At the end I couldn't just adjust ''firing angle'' I took apart transformer, count turns and order new one. If I remember correct I re-used laminations. To make things even worse there is no 220V version schematic of the original psu. It means a lot of testing and playing with values of resistors and capacitors in the gate circuit of triac.
If you desperately want to do it I might try to recover some more information what exactly I did. But please don't make me open M400. It is really messy job.
It might be I'm wrong about something. Just writing from my memory. It's been quite few years from this stunt...
I've done it with my M400. It is sn1755. There are 2 types of psu. Original and new version. At the end I couldn't just adjust ''firing angle'' I took apart transformer, count turns and order new one. If I remember correct I re-used laminations. To make things even worse there is no 220V version schematic of the original psu. It means a lot of testing and playing with values of resistors and capacitors in the gate circuit of triac.
If you desperately want to do it I might try to recover some more information what exactly I did. But please don't make me open M400. It is really messy job.
It might be I'm wrong about something. Just writing from my memory. It's been quite few years from this stunt...
Hi Tajzmaj,
Please don't even try that again. It is not a transformer like most people would think of. The unit works on current. The triac (as you noted) controls the energy that flows through the primary winding. I'm positive you got it to function, but it's dynamic operation scares the @#$%&@#*$^ out of me.
Even Carver Corp. didn't get it exactly right until a few years later. The M-400 new supply works well enough, but the newer supplies ran much more smoothly. So you can see why I am so uncomfortable with yours.
Keeping that in mind, Good show! You got it working. I just hope no one else heads down that road. A step-down transformer is the proper way to have it run on 230 VAC, especially if a qualified shop or technician can't be found.
-Chris
Please don't even try that again. It is not a transformer like most people would think of. The unit works on current. The triac (as you noted) controls the energy that flows through the primary winding. I'm positive you got it to function, but it's dynamic operation scares the @#$%&@#*$^ out of me.
Even Carver Corp. didn't get it exactly right until a few years later. The M-400 new supply works well enough, but the newer supplies ran much more smoothly. So you can see why I am so uncomfortable with yours.
Keeping that in mind, Good show! You got it working. I just hope no one else heads down that road. A step-down transformer is the proper way to have it run on 230 VAC, especially if a qualified shop or technician can't be found.
-Chris